U.S. Pat. Nos. 1,892,217 and 2,028,407, to R. J. L. Moineau, disclose a gear mechanism for use as a progressive cavity pump or motor. In a typical application of progressive cavity technology, the drilling of subterranean wells, a progressive cavity motor is used as a downhole motor to convert the energy of a flowing drilling fluid to mechanical power to rotate a drill bit.
In a progressive cavity pump or motor an interference fit between the external profile of the rotor and the internal profile of the stator provides a seal isolating the cavities of the pump or motor from adjoining cavities. The seal resists the fluid pressure which results from the mechanical pumping action, or from the conversion of fluid motion to mechanical energy in a motor. Because of the requirement for an interference fit between the rotor and stator, one or both of these components must be covered with a resilient, or dimensionally forgiving, material which also allows the pump or motor to pass or transfer abrasive particles and other objects carried along with the fluid. Historically, the resilient material has been provided on the interior of the stator.
The resilient material used for the stator introduces weaknesses into the operation of the pump or motor and shortens its operating life. Common elastomers have a temperature tolerances below that of most of the other components in the pump or motor, which are made of metal.
Mechanical resistance of the elastomer is also a concern because of the high fluid pressures generated in the cavities of the pump and motor. These high pressures, and the resulting reactive forces, result in a significant deflection and stress in the elastomer, particularly at the locations of the interferences between the rotor and stator. The friction resulting from the large forces existing between the rotor and stator generates a large amount of heat, which is deleterious to the desired characteristics of the elastomer, and thus deleterious to the performance and life of the pump or motor.
The stator is conventionally constructed by molding an elastomer, having the desired helical interior profile, within a cylindrical steel tube or housing. Due to the helical profile of the stator's internal surface, the radial thickness of the molded elastomer, between its inner surface and the inner surface of the metal tube, varies. If the heating of the elastomer is excessive, its properties will degrade. Elastomers are generally highly insulative, and thus inherently restrict conduction of the heat generated at the interface of the rotor and stator to the thermally conductive metal tube, where the heat can be dissipated, usually with the aid of a cooling system such as a liquid cooling system or exposed fins. The radially thicker sections of the elastomer are more insulative, and thus degrade faster than the radially thinner sections. Additionally, the high pressures produced during the operation of the pump or motor can deflect the thicker sections of elastomer to the extent that the interference between the elastomer and the rotor is overcome, and contact with the rotor is lost. This loss of contact results in a reduced operating efficiency, characterized by decreased speed in the case of a motor, and by decreased flow in the case of pump. In addition, heat generated by the operation of the pump or motor, in some cases acting in conjunction with heat from the environment in which the pump or motor operates, can distort the shape of the molded elastomer on the interior of the metal tube. Elastomers have a high coefficient of thermal expansion compared to the other materials used in the construction of a progressive cavity pump or motor. As a result of the varying thicknesses and relatively high thermal expansion of the elastomer, the radially thick sections tend to exhibit greater distortion than the thinner sections. The distortion results in a geometric stator profile drastically different from the intended profile, and hinders the operation of the pump or motor. The distortion of the stator profile can generate additional heat, which in turn causes further distortion of the stator profile. Because of such distortion the stator contributes rapidly to its own degradation and ultimate failure.
As a result of the previously mentioned degradation, the interior of the thicker sections also can become brittle, allowing a stator lobe to break or “chunk out” of the stator profile. In addition, the pressure acting in the chambers formed by the stator and rotor may exceed the strength of the elastomer, causing the stator lobe to deflect from its original shape, and may also cause a break or “chunk out”. These effects also degrade the efficiency of the pump or motor.
U.S. Pat. No. 6,309,195 describes a Moineau motor having a stator with a constant wall thickness. The stator is manufactured by a mechanical forming process in which the metal is bent locally to form a constant wall thickness in the outer steel structure, and in which the interior wall is covered by a thin wall elastomer. The dimensions of the stator produced by this forming method are limited, and more tolerance is required in the thickness of the thin wall elastomer. The patent alludes to the difficulty in maintaining the required twist tolerance. The outside of the casing is also contoured, making it more difficult to handle with the equipment commonly used to handle tubular articles in the drilling process. Machining of the outer wall of the casing to eliminate the contours would cause the wall thickness of the casing to be excessively small at some locations and comparatively thick at other locations.
Electrochemical machining has been used for various purposes. For example, U.S. Pat. No. 6,413,407 describes a process and apparatus for electrochemical machining (ECM) of flutes in the interior of a tube for use in a petroleum cracking furnace. However, so far as we are aware, ECM has not been used successfully in the production of the lobes in the interior of a stator of a progressive cavity device.